Loop construction for double array in Matlab

Question

I have a question about how can I construct a loop. I have a double array and I want to code that process.

This is the array where I want to apply the process and the length is 4x4 (I'm doing only the first, but the original was 4x4x3).

b1= (:,:,1);

I want to apply for every value this process:

1.- Create a vector every time, with the information of every value of the array.

ma= 0;
 for p=(136:136)
 ma(p)=b1(1,1,1);
 end
 for p=(312:2151)
     ma(p)=0
 end
 ma= ma';

Then, I have to do with the last result the following process (I've defined the variables before). spout1_a= spb1y.ma; spout1_b= spout1_a./spsum_pesos1; spout1_c= sum(spout1_b);

The problem is I know how can to do it for one value (the first one), but not with all of them. How can I do it?

Greetings and thank you so much,

Emma

EDIT

b1= Refl(:,:,1);

load sp1.txt;

spb1y= sp1(:,1);
spsum_pesos1= sum(spb1y);
output = cell(length(banda1), 5); % this works well

 for i = 1:numel(b1)
    ma = zeros(2151,1);
    ma(136) = output(i);
    spout1_a= spb1y.*ma;
    spout1_b= spout1_a./spsum_pesos1;
    spout1_c= sum(spout1_b); % I want to save that result on every value of the matrix
end

Am I writing this last part well?

What is the logic behind running a for loop from 136 to 136? It's just one iteration... or perhaps you're trying to achieve something else? — Eitan T
– Eitan T, Commented Nov 8, 2012 at 12:48
Hi Eitan, I want to do this because the cell array spout1_a are on this 'position'. — user1578688
– user1578688, Commented Nov 12, 2012 at 7:59

Eitan T · Accepted Answer · 2012-11-08 12:52:43Z

1

Here are a few pointers for coding in MATLAB:

Preallocate: allocate memory prior to the loop so that variables don't grow in dimensions inside the loop. This causes MATLAB to dynamically allocate memory in each iterations, which can often slow execution time significantly.
Vecotrize: try to use loops only where you have to. In many cases, you can use vectorized operations instead, which are a lot faster.

With that said, you can try this:

% # Iterate over each value in 'b1'
for i = 1:numel(b1)

    % # Create a vector 'ma'
    ma = zeros(2151, 1);
    ma(136) = b1(i);

    % # Do some more calculations with 'ma' ...
    spout1_a = ma;
    spout1_b= spout1_a ./ spsum_pesos1;
    spout1_c= sum(spout1_b);
end

I'm not sure what you want to achieve, but this code does exactly what the question describes.

edited Nov 8, 2012 at 12:52

answered Nov 8, 2012 at 12:47

Eitan T

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1 Comment

user1578688 Over a year ago

Thanks for your reply. I don't know why but it doesn't work at all. I'm going to paste it editing the post.

Dennis Jaheruddin · Accepted Answer · 2012-11-08 12:44:19Z

1

The simple solution would be to add an index q to every variable and then loop from q=1:3

b(q)= Matrix(:,:,q)
ma(p,q)

etcetera

answered Nov 8, 2012 at 12:44

Dennis Jaheruddin

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1 Comment

user1578688 Over a year ago

Thanks Dennis, but the problem is I have to create in a 'exactly position', this is the reason of the 136:136 ! But thank you so much! ;)

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